1. Technical Field
This disclosure relates to an image forming apparatus, and particularly relates to an image forming apparatus having a recording head that forms an image by ejecting liquid droplets.
2. Description of the Related Art
Image forming apparatuses (e.g. printers, fax machines, copiers, and multifunction machines having functions of these machines) are known that perform image formation by ejecting a liquid (a recording liquid) such as ink onto a medium using, for example, a liquid ejection device while transporting the medium. The liquid ejection device comprises a recording head including a liquid ejection head for ejecting droplets of the recording liquid (ink). It is to be noted that the term “medium” as used herein is also referred to as a “sheet”, which may be paper or may be made of other materials. The terms “to-be-recorded medium”, “recording medium”, “transfer material”, and “recording sheet” may be used as synonyms for the term “medium”. The terms “recording”, “printing”, and “imaging” may be used as synonyms for the term “image formation”.
The term “image forming apparatus” as used herein indicates an apparatus that forms images by ejecting liquid onto media such as paper, strings, fibers, cloth, leather, metal, plastic, glass, wood, and ceramics. The term “image formation” as used herein indicates not only forming images that have meanings, such as characters and figures, on a medium, but also forming images that do not have meanings, such as patterns, on a medium, and applying a material having desired properties onto any position on a medium. The “ink” is not limited to an ink but may include any liquid such as liquid that are commonly called ink, recording liquid, DNA samples, resist, resin materials, patterning materials, and materials that have desired properties (e.g., light emitting properties, light blocking properties, conductive properties, fixative properties, glossy properties, and liquid absorbability).
In image forming apparatuses using such an ink, because an image is formed using the ink, a certain amount of time is required for the ink ejected on a recording medium to be dried. Therefore, the recording medium on which an image is formed is held in a waiting mode inside the apparatus until the ink ejected on the recording medium is dried. In the case of duplex printing, the recording medium is ejected onto a discharge tray and then is fed again.
For example, Japanese Patent Registration No. 3109529 (corresponding to Japanese Patent Laid-Open Publication No. 4-255354) discloses an inkjet printer that includes a unit that delays discharge of a recording sheet to a discharge tray for a set period of time if the dot density of the previous determination result is higher than a predetermined value.
Japanese Patent Laid-Open Publication No. 2000-001010 (corresponding to Japanese Patent Registration No. 3681093) discloses an inkjet recording apparatus that, after printing one side of a sheet, discharges at least a part of the sheet outside the apparatus to secure drying time, and then prints the other side of the sheet.
Japanese Patent Laid-Open Publication No. 2006-082546 discloses an image forming apparatus that causes a recording medium, on which an image is formed by a recording head, to wait until the recording medium becomes difficult to be curled while holding the upper side and the lower of a part of the recording medium, and then discharges the recording medium.
Japanese Patent Laid-Open Publication No. 2003-248349 discloses an image forming apparatus, such as a laser printer that forms an image using toner, that delays discharge of a sheet of poor fixative properties such as an OHP sheet by causing the sheet to be held at a discharge port until the temperature of the sheet decreases.
Japanese Patent Laid-Open Publication No. 2005-292651 discloses a fixing device that includes plural fixing units for fixing a toner image onto a sheet, a bypass path for transporting the sheet not to pass through at least one of the fixing units, a main path for transporting a sheet to pass through at least one of the fixing units, and a path switching unit disposed at a branching point of the main path and the bypass path and configured to select one of the paths to which the sheet is guided. The main path and the bypass path join together at a joining point. The time required to transport the sheet from the branching point to the joining point via the main path is substantially equal to the time required to transport the sheet from the branching point to the joining point via the bypass path.
Highly viscous ink tends to be used in the image forming apparatuses in order to achieve high-speed and high-quality image printing on plain paper. Especially, in the case of pigment ink using organic pigment, carbon black or the like as colorant, the pigment is not soluble in water unlike dye. Therefore, the pigment ink is normally used as aqueous ink mixed with dispersant in which the pigment is stably dispersed by the dispersant. Such aqueous ink generally has high viscosity (5 mPa·s or greater). If this aqueous ink is used for forming an image on plain paper (recording medium), although the ink dries quicker than dye ink, the recording medium is likely to be curled.
In the case of dye ink, water penetrates to reach the back side of the recording medium, so that the difference in water content between the front side and the back side of the recording medium is small. Accordingly, although it takes time for the ink to dry, curling due to the difference in water content between the front side and the back side of the recording medium is relatively not likely to occur. On the other hand, in the case of pigment ink, it does not take much time for the ink on the recording medium to dry due to its quick drying properties, but it takes time for water to penetrate into the recording medium, so that the difference in water content between the front side and the back side of the recording medium is great. Accordingly, curling is likely to occur due to the difference in water content between the front side and the back side of the recording medium. Therefore, if the recording medium becomes curled, because the ink dries quickly, the ink is cured while the recording medium remains curled.
If the curled medium is transported without being decurled, the medium may become jammed or be folded due to low discharge stability, resulting in reducing the quality of the printed medium.
If the liquid ejection type image forming apparatuses are configured to discharge sheets with their image sides down in the same manner as laser printers in order to arrange the output sheets in the same order as the original sheets (in the order of page number from the first page to the last page), because the ends of the sheets are curled upward, it is difficult to stack the curled sheets in a discharged sheet stacker compared to image forming apparatuses configured to discharge sheets with their printed sides up.
Moreover, the sheets are further curled after being discharged into the discharged sheet stacker.
These problems create a growing need for a solution to prevent curling of sheets and contamination of the sheets. In the case of the related-art techniques described above, while a sheet is held in a waiting mode to be decurled and dried, recording of the next sheet is not performed, so that the throughput decreases, resulting in reducing the productivity.